The present invention relates to a method and apparatus for cooking comminuted cellulosic fibrous material to produce cellulosic pulp. Especially preferably, the invention relates to an alkaline batch cooking process, i.e., to a so called Batch cooking process.
Alkaline cooking processes, in particular the sulphate or kraft process, have conventionally been performed in such a way that the wood matter, i.e. the chips, and the cooking liquor are added into a cooking vessel, the temperature of which vessel is then raised to the cooking temperature for a given period of time, whereby lignin is detached from the wood matter, the end result being chemical pulp and waste liquor. In the sulphate process, white liquor, i.e., a mixture of the active cooking chemicals sodium hydroxide (NaOH) and sodium sulfide (Na2S), is used as the cooking liquor. The concentration of the active cooking chemicals is typically given in terms of xe2x80x9ceffective alkalixe2x80x9d (EA) expressed as g/l as equivalent NaOH. The waste liquor is typically referred to as xe2x80x9cblack liquorxe2x80x9d which contains some amount of residual cooking chemicals, usually below 10 g/l EA as NaOH as well as dissolved wood material.
Modifications have been made to the conventional batch sulphate process, including the known SUPERBATCH and RDH batch cooking processes. By reusing waste liquor, i.e. black liquor, generated in the cook a good heat economy and a low cooking Kappa number can be achieved with these processes. However, mainly due to the reuse of the black liquor, the processes involve a great problem, namely inferior bleachability of pulp, that is, the pulp produced by these processes typically requires more bleaching chemical to obtain a comparable brightness compared to pulps produced from other methods.
In a cooking method according to our invention, it has been possible to eliminate the problem of bleachability by further development of the kind of process mentioned. This has resulted above all in better bleachability, but in addition, the yield and relative strength of the pulp has also improved. It has been noted that the actual problem is the dissolved organic material contained in the black liquor, in other words, the organic material which is dissolved from the wood matter and enters into the liquor. In modified batch processes mentioned above, organic material is typically returned both to the pre-treatment and the actual cook to such an extent that the pulp produced has inferior bleachability, yield and strength compared to the pulp produced by the present invention.
Said problem has been removed by a method according to the present invention, which method comprises the steps of supplying the wood matter into the digester; pre-treating the wood matter in the digester with a first liquor; displacing the first liquor by a second liquor; directing said displaced first liquor to a further treatment; treating the wood matter with the second liquor; displacing the second liquor with a wash liquid; and removing the cooked pulp from the digester; and which is characterized by such a displacement of the second liquor that a strong black liquor BLS is obtained, having an alkalinity of 15-40 g/l as NaOH, and by the supply of strong liquor obtained in this way into the digester at a temperature of below 140xc2x0 C., preferably below 130xc2x0 C., and most preferably below 120xc2x0 C.
Another embodiment of this invention consists of a method of cooking comminuted cellulosic fibrous material to produce cellulose pulp in one or more batch digesters, consisting of the following steps: a) introducing the material to the digester; b) introducing a first liquor (BLS) having a first effective alkali concentration to the digester; c) treating the material in the digester with the first liquor; d) displacing the first liquor from the digester with a second liquor (WL/BLW) having a second effective alkali concentration, to produce a third liquor, displaced from the digester, having a third effective alkali concentration lower than the first effective alkali concentration; e) treating the material in the digester with the second liquor(WL/BLW); f) displacing the second liquor from the digester with a fourth liquor (W) having a fourth effective alkali concentration, lower than the third alkali concentration, to produce a fifth liquor, displaced from the digester, having a fifth effective alkali concentration, lower than the second effective alkali concentration; and g) removing the treated material from the digester; characterized in that the fifth liquor (BLS) has a fifth effective alkali concentration of between 15-40 g/l, preferably, between 20-35 g/l, most preferably, between 25-35 g/l expressed as NaOH and the fifth liquor comprises at least some of the liquor used as the first liquor in step (b).
In a preferred embodiment the fifth liquor is cooled to a temperature below 140xc2x0 C. prior to its use as the first liquor in step (b). The temperature of the fifth liquor (BLS) may be even cooler, for example, below 120xc2x0 C. or even between 80-100xc2x0 C. This fifth liquor may be cooled by passing it in heat exchange relationship with a sixth liquor, for example, white liquor (WL) or weak black liquor (BLW), which can be used individually or in combination as the second liquid or the fourth liquid.
The fourth liquid (W) may be washer filtrate, condensate or any other liquid stream having low alkali content (that is, less than 5 g/l as NaOH) and low dissolved organic content.
The third effective alkali concentration of the third liquor, that is the liquor displaced by the second liquor in step (d) is typically less than 10 g/l, as NaOH and can be less than 5 g/l as NaOH. The duration of step (c) is typically between 20-120 minutes, preferably between 30-90 minutes.
The method described above may also include an additional step (h) between steps (f) and (g) wherein a seventh liquor (BLW) is displaced from the digester by the fourth liquor (W) after the fifth liquor (BLS) is displaced. This seventh liquor (BLW) will have a sixth effective alkali concentration which will be less than the fifth alkali (BLS) concentration. This seventh liquor (BLW) may be used with the fifth liquor (BLS) for the first liquor in step (b) or with the second liquor (WL) of step (d) or both.
The present invention also includes an apparatus for performing a batch cooking process, comprising several digesters (10) and means (12, 14, 16, 18, 26, 28, 42, 48, 50, 62, 70) for storing and recirculating different liquors in the process, characterized in that the apparatus further comprises means (20, 40, 60, 66) for cooling the liquor displaced from the digester (10) prior to the supply of the liquor into the digester (10) and prior to the raising of the temperature in the digester. The cooling means may be a heat exchanger (20, 40, 60) or a flash tank (66). The heat exchanger (20, 40, 60) may be connected to a pipe line (26, 64) between a liquor tank (12) and the digester(10). The heat exchanger (20) may also be connected to a pipe line (46) between the digester (10) and the liquor tank (42). The heat exchanger (60) may also be connected to a pipe line (64) between a liquor tank (62) and the flash tank (66). The flash tank (66) may be connected by a pipe line to a steaming vessel (68) or a chip hopper. The apparatus may further comprise a pump (74) arranged in connection with the chip hopper or steaming vessel, by means of which pump the chips are transferred into the digester (10).